A pilot study of the influence of residential HAC duty cycle on indoor air quality

Abstract A simple methodology was developed to collect measurements of duty cycle, the fraction of time the heating and air conditioning (HAC) system was operating, inside residences. The primary purpose of the measurements was to assess whether the HAC duty cycle was related to reductions in indoor particulate matter (PM) concentrations. A miniature temperature logger placed on an HAC outlet vent monitored changes in temperature as the system cooled or heated the residence. Temperature step changes signaling duty cycle periods were identified using spreadsheet macros. Parallel measurements of 24-h integrated air exchange rates (AERs) and indoor PM 2.5 and PM 10 concentrations were collected to determine relationships with the duty cycle. The mild temperatures (mean=18.7°C) present during the Spring season of the RTP PM Panel Study and personal comfort preferences caused low and variable daily duty cycles (mean=0.061, std. dev.=0.054) in both heating and cooling mode. Warmer ambient temperatures during the Fall season of the Tampa Asthmatic Children's Study (TACS) resulted in cooling-only HAC operation, with a higher mean duty cycle of 0.21 (std. dev.=0.11). Statistically significant linear relationships were observed between daily average duty cycle and the ambient temperature for both studies. Duty cycle exhibited a strong diurnal pattern commensurate with ambient temperature fluctuations. Duty cycles were positively associated with the residence AERs for heating-mode operations, but negatively associated when operating in cooling mode. Personal preferences contributed to the variability in the relationship between duty cycle and AER. The relationship between duty cycle and PM 2.5 or PM 10 indoor–outdoor ratios were not statistically significant. The association of duty cycle with indoor–outdoor ratio was confounded by the short time span (mean of 10.3 min for the TACS) of HAC system operation and the presence of strong indoor sources altering the indoor concentration levels.